Electro-optical display apparatus for signalling warning of predetermined condition ambient compensated

ABSTRACT

A display device signals a warning for the existence of a condition, such as low fuel in a motor vehicle and employs an element in which certain optical properties of a liquid crystal composition change when a threshold voltage is applied across it in response to a controlling sensor. The sensor is located in a fluid reservoir and exhibits a first resistance level while being cooled by the fluid and a second resistance level when out of contact with the fluid. The sensor is electrically coupled with the composition so that the voltage gradient impressed across the composition is dependent upon the resistance level of the sensor. Improved contrast and ambient temperature compensation is achieved in certain of the embodiments by mounting the liquid crystal composition in heat transfer relation with an electrical resistance filament.

'3,674,342 7/1972 Castellanoetal. ..'...'.340/324R RR 398a775 -x pa Y l 5". ilntted States 1 7 5 Epstein Nov. 5, 1974 ELECTRO-OPTICAL DISPLAY APPARATUS Primary ExaminerJohn W. Caldwell FOR SIGNALLHNG WARNING OF Assistant ExaminerMarshall M. Curtis PREDETERMHNED CONDITION AMBIENT Attorney, Agent, or Firm-John A. Haug; James P. COMPENSATED McAndrews; Edward J. Connors, Jr.

[75] Inventor: Henry David Epstein, Cambridge,

Mass [57] ABSTRAT [73] Assignee: Texas Instruments Incorporated, A display device signals a warning for the existence of Dallas a condmon, such as low fuel 1n a motor vehicle and employs an element in which certain optical properl Flledi 1972 ties of a liquid crystal composition change when a [21] AppL NOJ 295,641 threshold voltage is applied across it in response to a controlling sensor. The sensor is located in a fluid reservoir and exhibits a first resistance level while being 340/244 350/160 LC cooled by the fluid and a second resistance level when [51] int. Cl. G02i 1/16 out of ontact with the fluid, The ensor is electricany Field 0i Search 340/244 R, 244 C, 324 R, coupled with the composition so that the voltage gra- 340/324 50/1 L 73/295 dient impressed across the composition is dependent upon the resistance level of the sensor. Improved conl l References Cited trast and ambient temperature compensation is UNITED STATES PATENTS achieved in certain of the embodiments by mounting 3,350,710 10/1967 Bridges 340 244 R the liquid crystal composition in heat transfer relation 3,499,112 3/1970 fleilmei with an electrical resistance filament.

13 Claims, 9 Drawing Figures amena- 012 IN; sec/244R ATENIED ROY 5 19h smear a ELECTRO-OPTICAL DISPLAY APPARATUS FOR SIGNALLING WARNING OF PREDETERMINED CONDITION AMBIENT COMPENSATED This invention relates to warning display apparatus and more particularly to electro-optical display apparatus for signalling a predetermined condition, particularly the condition of low liquid, for example, brake fluid, oil or fuel, in a reservoir of a motor vehicle.

In recent years the public has become increasingly concerned with the safety of consumer products and particularly so with the safety of automobiles. A great amount of effort and expense has been expended to provide features to make automobiles safer and more convenient with concomitant increase in cost of the vehicle. Thus as the number of safety-features provided increases the more important it becomes to achieve cost effectiveness in order to keep the total cost of the vehicle from increasing at a prohibitive rate. If certain devices can be replaced by less costly but functionally identical or superior devices, then this saving can be devoted to other new features. For instance, it is known to provide warning lamps to indicate the presence of a certain condition, such as low fuel. Normally this is accomplished by employing a sensor of some type, such as a float in the fuel tank, which is adapted to send a signal when the fluid level drops to a predetermined point. The signal is amplified in some way, as through a thermal relay, and turns on a lamp indicating the low fuel condition. The present invention is an improvement on this type of system and eliminates the need for any type of amplifier while still providing optical indication of the particular condition.

It is therefore an object of the invention to provide apparatus of the type described above for giving warning indication of the existence of a predetermined condition which is not only reliable but is also inexpensive to produce. Another object is the provision of apparatus which has few parts, takes little power to operate and is especially suitable for vehicular use. Yet another object is the provision of visual indicating apparatus having good optical contrast without using amplifiers of any kind.

Briefly, in accordance with the present invention, apparatus particularly suited for motor vehicles for signalling a warning of a predetermined condition includes a display element comprising a first opticallytransmissive substrate and a first optically-transmissive and electrically conductive coating on a face ofthe first substrate. A second substrate with an electrically conductive coating on a surface thereof is positioned such that the substrates are adjacent and substantially equidistantly spaced apart. A liquid crystal composition is placed in the space between the substrates, the composition having a threshold electrical field which when exceeded will change certain optical properties of the composition. Means are provided for impressing 21 voltage level between the coatings which depends on the existence of the condition. The display being an inherently low power consuming device is advantageously used with a condition responsive sensor for providing a read out signal of the existence of a condition. By way of example, a sensor formed of positive temperature coefficient of resistivity material is placed in the master cylinder of the brake system and is adapted to be inundated by the-fluid. The sensor is electrically coupled to the voltage source and generates heat due to electrical current passing therethrough. As long as the sensor is immersed in the fluid the heat is conducted away from the sensor thereby keeping it at a low temperature. The sensor has a resistivity-temperature relationship such that at temperatures below an anomaly it exhibits a low level of resistance and at temperatures above the anomaly the resistance rapidly increases, several orders of magnitude for just a few degrees Centigrade. In the event that the sensor becomes exposed due to lowered level of brake fluid caused by leakage for example heat is not transmitted away from the sensor as quickly as it is generated and thus the temperature of the sensor increases until it is above the anomaly at which point the sensor goes into a high resistance mode. This change in resistance causes a concomitant change in the voltage level impressed across the display element. For certain nematic fluids in one instance the voltage level across the element is above the threshold electrical field causing light passing therethrough to scatter while in the other instance the voltage level across the element is below the threshold electrical field so that the nematic composition is clear and light passing therethrough is not scattered. An electrical resistance element, such as a lamp filament, may be placed in heat transfer relation with the display in order to improve contrast and to extend the useful temperature range at which the liquid crystal composition may be effectively employed.

The invention accordingly comprises the elements and combinations of elements, features of construction and manipulation and arrangement of parts, all of which will be exemplified in the structure hereinafter described, and the scope of the application of which will be indicated in the appended claims.

The above and other important objects of the invention will become apparent from the'following description and the accompanying drawings illustrating several embodiments of the invention in which:

FIG. 1 is a schematic circuit diagram showing a resistor coupled in series between a voltage source and a sensor with the display element connected across the resistor in accordance with the present invention;

FIG. 2 is a side view of such a display element mounted in the dashboard area of a motor vehicle;

FIG. 3 is a side view of the display element shown in FIGS. 1 and 2;'

FIG. 4 is a front view of an etched substrate which forms a part of a display element;

FIG. 5 is a side view similar to FIG. 2 but shows a modified version of the invention;

FIG. 6 is a schematic circuit diagram similar to FIG. 1 but showing an alternative arrangement in which the display element is connected across the sensor;

FIG. 7 is a side view similar to FIG. 2 but showing a modified version of the invention;

FIG. 8 is a side view similar to FIG. 2 but showing a modified version of the invention; and

FIG. 9 is a front view of a substrate on which an electrode coating is formed in the shape of a symbol, the substrate forming part of a display element.

Similar reference characters indicate corresponding parts throughout the views of the drawings.

Dimensions of certain of the parts as shown in the drawings may have been modified or exaggerated for the purpose of clarity of illustration.

Display element 10 comprises a thin layer of liquid crystals which has certain optical properties in the absence of any electrical field and certain different optical properties when an electrical field at or above a threshold level is applied thereacross. The existence of liquid crystals has been known for many years and have generally been classified into three categories; nematic, cholesteric and smectic. In general liquid crystals are organic molecules which have an anistropic shape, having a much greater length than cross section. While other liquid crystalline substances can be used such as cholesteric in which circular dichroism is characteristic effecting the color of transmitted and reflected light, it is preferred in this invention to make use of the dynamic scattering characteristic of certain nematic liqaids in which the liquid crystals tend to align in a direction not parallel with an applied electric field and have a finite conductivity in such liquids. According to one explanation, normally the average direction of the long axis of the molecules is nearly parallel and allows light to pass therethrough when no voltage is applied. However, when threshold voltage is applied the molecules of some nematic materials have dipole moments at an angle with respect to the applied field and ions traveling in the liquid material disrupt the normal orientation by causing some molecules to line up in the direction of the local field around the ion thereby resulting in discontinuity in the molecular orientations and light which otherwise would pass through is scattered resulting in a degree of opaqueness or translucency.

Referring to FIG. 1, display element is shown connected across a resistance 16 which in turn is serially connected to a voltage source 12, such as a battery of the vehicle, and a sensor 14. The sensor 14 is responsive to a condition, such as temperature. In the illustrated embodiment a positive temperature coefficient of resistance thermistor is employed. This thermistor has a low resistance mode at temperatures below an anomaly, e.g. 80C., and a high resistance mode at temperatures above the anomaly. As temperature is increased above the anomaly, the resistance increases several orders of magnitude for just a few degrees Centigrade. An example of such a sensor is a barium titanate doped with a rare earth. Lamp 18 is coupled across the voltage source and on-off switch 20, which conveniently may be the ignition switch, is provided to energize the system.

Sensor 14 is mounted in a fluid reservoir 22 which may, for example, be the master cylinder in the brake system. the crank case or fuel tank. Upon energization of the system by closing switch current passes from voltage source 12 through resistor 16 and sensor 14. When the sensor is immersed in the fluid heat generated in sensor 14 is transferred into the fluid thereby keeping the resistance of the sensor low. This causes most of the voltage drop to occur across display element l0 and resistor 16, substantially above the threshold level of the nematic liquid crystal which is in the order of6 volts for 0.5 mil thick element, the resistance value of resistor 16 being substantially higher than the resistance value of sensor 14 at temperatures below its anomaly. Thus when the sensor is immersed the display unit is translucent and scatters light passing through or reflecting from it. However. once the fluid level drops sufficiently to expose sensor 14, heat is generated in the sensor due to the current passing therethrough at a faster rate than can be transferred away from it to the surrounding air. This causes the temperature to rise until it is above the anomaly above which point the resistance of the sensor rapidly increases to a much higher level than resistor 16. In this condition the voltage drop across the display is reduced below the threshold value and the nematic liquid crystal becomes transparent and permits light to pass therethrough.

FIG. 2 illustrates one way such a display is employed. Display element 10 is mounted in a dashboard section of a motor vehicle with lamp 18 located on one side of the element and at an angle to the faces ofthe element. A tube 24 is preferably placed between lamp l8 and element 10 to collimate the light and direct it to the element. The tube is blackened to absorb stray light waves. On the other side of element 10 dashboard section 26 is located which may be a padded portion also of dark material to absorb incident light striking it. Leads 28 and 30 extending from the display element 10 connect the element into the electrical circuit while leads 32, 34 connect lamp 18 in the circuit. As seen in FIG. 3, display element 10 comprises two substantially parallel optically transmissive substrates, such as glass, having respective electrically conductive coatings 40, 42 on the inside faces thereof. Conductive coating 42 which may be formed of tin oxide, is optically transmissive and for reasons that will become apparent below, conductive coating may be optically reflective or transmissive; however, as used in FIG. 2, both coatings are optically transmissive. An electrically insulative spacer 44 may be used to maintain the proper distance between substrates 36, 38 and assure parallelism. Occupying the space between conductivecoatings 40, 42, is a composition of nematic liquid crystal. Although enlarged in the drawings to better illustrate this structure of the display element it should be realized that the distance between the substrates is quite small, for example 0.5 mils.

When the sensor is immersed in fluid as described above, the voltage drop across display element 10 is sufficient to cause the nematic liquid crystals to scatter light, thus appearing frosted. Etched on coating 42 side of substrate 38 is a symbol indicative of the warning to be given, such as OIIJ as shown in FIG. 4 by dashed lines 47. When the light ray 48 is scattered there is no significant contrast between the etched portion 47 and the remainder of the substrate the entire surface appearing frosted; however, when the sensor 14 is exposed due to low liquid level the voltage drop occurs primarily across the sensor since it goes into the high resistance mode. Since the voltage drop across the display element is below the threshold level of the nematic liquid crystal, light ray 50 is not scattered and passes directly through display element 10 thereby resulting in a significant optical contrast between etched portion 47 and the remainder of substrate 38. Thus the observer, indicated at 52, warned of the low liquid condition, can take remedial action.

It will be noted that lamp 18 is so oriented that the light rays therefrom form an angle with a line normal to the face of display element 10 approximately to achieve optimum results.

When sufficient light is available within the vehicle, either from the sun during the day or a dashboard light during the night, lamp 18 can be eliminated as indicated in FIG. 5. In this instance, conductive coating 40 on substrate 36 of display element 10 is made to reflect light rather than to transmit as does conductive coating 42 in substrate 38. Thus when sensor 14 is immersed in fluid and in the low resistance state the voltage drop across the display element is above the threshold level of the nematic liquid crystal. Therefore light ray 56 striking display element is scattered with very little reaching the eye of observer 52. When sensor 14 is exposed it goes into the high resistance state as explained above so that the voltage drop across the display unit 10 is below its threshold level so that light ray 58 passes through the liquid crystal composition and is reflected from conductive layer 40 to the observers eye thereby giving him notice of the presence of the particular condition, i.e. low liquid level. To enhance the visibility of the warning a symbol can be etched in substrate 38 as in the previous embodiment to effect optical contrast when the nematic liquid crystals transmit light therethrough.

FIG. 6 shows a schematic circuit diagram similar to FIG. 1 in which display element 10 is electrically coupled across sensor 14. A current limiting resistor 54 having a value considerably higher than that of sensor 14 when sensor 14 is in'the low resistance state may be placed in series with the sensor 14 and display element It). In this arrangement when sensor 14 is immersed in the fluid in reservoir 22 and with the sensor in the low resistance state, the voltage gradient across the display element is less than the threshold level thus the nematic liquid crystal would be clear permitting light to be transmitted therethrough. Display element 10 seen in FIG. 7 is provided with optically transmissive substrates and optically transmissve, electrically conductive coatings on both opposite sides of the element. Thus when sensor 14 is immersed light ray 60 is transmitted from lamp 18 through display element 10 to observer 52 indicating that sufficient fluid is in the reservoir. When the fluid level drops below sensor 14 permitting it to heat up and go into the high resistance state, the voltage appearing across the display elementis above the threshold level of the crystal liquid therefore scattering light ray 62 giving observer 52 indication of low liquid level.

Another benefit to be derived from lamp 18 in both the FIG. I and FIG. 6 circuits is that heat transmitted from the lamp to display element 10 enables a wider variety of liquid crystals to be used. That is most liquid crystal compositions have a limited temperature range in which they are nematic. Many known materials solidify approximately between l5C. and to +20C. with the effective minimum operating temperature being somewhat higher. That is, switching becomes quite slow, 5 to 10C. above the freezing point. Even though certain liquid crystals are not mesomorphic over a particular ambient temperature range, the heating effect of the lamp raises the lower end of the temperature range to which element 10 is subjected during operation in effect providing ambient compensation. In this respect, the embodiment depicted in FIG. 8 is especially effective both in providing improved contrast and in extending the useful temperature range in which the display may effectively be employed. Unit 18 comprises a housing formed with an open end which is closed by display element 10. Display element 10 is similar to that shown in FIG. 3. Electrical resistance element in the form of lamp filament 33 is located adjacent the open end of housing 18 in heat transfer relation with display element 10. Leads 32, 34 are provided for energizing filament 33 while leads 28, 30 are provided for energizing the display element 10. Energization of filament 33 not only provides back lighting for the liquid crystal display but also provides sufficient heat for the liquid crystal composition to enable it to be effective even though unit 18 as a whole is subjected to low ambient temperatures present, for example, in winter conditions. Thus unit 18 forms a self contained ambient temperature compensated display unit which can conveniently be inserted in a dashboard aperture obviating the need for other light collimating means or the like. The upper end of the useful temperature range, that is when the material becomes isotropic, is sufficiently high that special means need not be taken for most display applications to extend that portion of the useful operating temperature range.

In FIG. 9 display element 10 is provided with substrate 38 having deposited thereon optically transmissive, electrically conductive coating 42 in the shape of a symbol e.g. gas. When the voltage level appearing across the display 10 is below the threshold level the symbol will not be seen since light passes through the entire substrate in approximately the same way; however, when the voltage across the display element is above the threshold level, that portion of the nematic liquid crystal located between the conductive coating 40 and the shaped electrode 42 gas" will scatter light rendering the-symbol translucent and easily readable.

Use of a display element employing liquid crystals as described above is particularly advantageous since such a low energy level is required to effect the orientation of the molecules, that is, the material is unable to sustain a shear stress. Another important advantage over emitted light displays is a contrast ratio which is independent of the ambient light intensity. A liquid crystal display will be equally visible at all light ambients with contrast ratios of 20:] or better.

Since the liquid crystal display is such a low power consuming element, it is particularly useful with temperature sensitive thermistors. For example, if thermistors 14 formed of positive temperature coefficient ma-- terial were used with a standard bulb in the same way the display element is employed in the invention, too much current would be drawn and the thermistor 14 would remain only in the high resistance mode, that is the current drawn by the bulb would cause the thermistor 14 to self heat into the high resistance mode, Thermistor 14 could also take the form of an element having a negative temperature coefficient which, when used with the display element, can provide a direct read out signal.

It will be appreciated that various other liquid crystal arrangements and optical characteristics could be utilized such as linear dichroism, i.e. the preferential adsorption of one direction of polarized light in nematic liquid or circular dichroism in cholesteric liquid crystals.

As many changes could be made in the aboveconstructions without departure from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings, shall be interpreted as illustrative and not in a limiting sense, and it is also intended that the appended claims shall cover all such equivalent variations as come within the true spirit and scope of the invention.

It is to be understood that the invention is not limited in its application to the details of construction and arrangement of parts illustrated in the accompanying drawings, since the invention is capable of other embodiments and of being practiced or carried out in various ways. Also, it is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation.

l claim:

1. Automotive warning display apparatus comprising:

a sensor having a steeply sloped positive temperature coefficient of electrical resistance;

a housing having an open end;

an electrical resistance lamp element mounted within the housing having a portion adjacent the open end thereof;

a display element received in and closing the open end of the housing and in heat transfer relation with the electrical resistance lamp element includin a fir t optically transmissive substrate having an optically transmissive and electrically conductive coating on a face thereof,

a second substrate having an electrically conductive coating on a face thereof, the substrates being positioncd such that the coatings are adjacent and substantially parallely spaced,

a liquid crystal composition occupying the space between the coatings on the substrates, the composition having a threshold voltage level at which an optical characteristic of the composition is changed; and voltage impressing means for impressing a voltage gradient between the coatings, the sensor being electrically coupled to the means for impressing a voltage gradient so that current is conducted through the sensor, the sensor normally operating in one oftwo temperature ranges. the resistance level of the sensor controlling the magnitude ofthe voltage gradient, the voltage impressing means including means to electrically energize the electrical resistance lamp element, heat from the lamp element being transferred to the display element to provide ambient temperature compensatlon.

2. Apparatus according to claim 1 in which the liquid crystal is nematic and the optical characteristic which changes upon application of threshold voltage level is a light transmitting characteristic whereby light passing therethrough is relatively unimpeded when the voltage level is below the threshold and is dynamically scattered when the voltage level is at least as high as the threshold level.

3. Apparatus according to claim 2 in which the sensor comprises a thermistor formed of material having a positive temperature coefficient of resistivity, the sensor having a first low resistance state at temperatures below an anomaly and a second high resistance state at temperatures above the anomaly.

4. Apparatus according to claim 2 in which one ofthe substrates is etched with a desired symbol.

5. Apparatus according to claim 3 in which the sensor is electrically coupled in series with the display element, and a resistor is coupled across the display element to permit current to flow to the sensor from the voltage impressing means.

6. Apparatus according to claim 3 in which the sensor is electrically coupled across the display element.

7. Apparatus according to claim 3 further including a fluid reservoir, the sensor mounted in the reservoir such that it is completely immersed in the fluid when the reservoir is filled but is exposed when the fluid level falls beyond a predetermined limit.

8. Apparatus according to claim 2 in which one of the conductive layers of the display element is formed in the shape of a desired symbol.

9. A display unit comprising:

a housing having an open end;

an electrical resistance lamp element mounted within the housing having a portion adjacent the open end thereof; an optically transmissive lens received in and closing the open end of the housing and in heat transfer relation with the electrical resistance lamp element, the lens formed of a first and second parallely spaced optically transmissive substrates, the substrates each having an optically transmissive and electrically conductive coating on a face thereof, a liquid crystal composition occupying the space between the substrates, the composition having a threshold voltage level at which an optical characteristic of the composition is changed; and

voltage impressing means connectable to the coatings and to the electrical resistance lamp element for impressing a voltage gradient between the coatings and for permitting current to flow through the electrical resistance lamp element, heat from the lamp element being transferred to the lens to provide ambient temperature compensation.

10. A display unit according to claim 9 in which the liquid crystal composition is a temperature sensitive nematic composition.

11. Apparatus according to claim 9 further including a sensor electrically coupled to the lens coatings, the sensor comprising a thermistor whose resistance varies with temperature.

12. Apparatus according to claim 11 in which the thermistor is formed of material having a positive temperature coefficient of resistivity, the thermistor having a first low resistance state at temperatures below an anomaly and a second high resistance state at temperatures above the anomaly.

13. Apparatus according to claim 11 in which the thermistor is formed of material havinga negative temperature coefficient of resistivity. 

1. Automotive warning display apparatus comprising: a sensor having a steeply sloped positive temperature coefficient of electrical resistance; a housing having an open end; an electrical resistance lamp element mounted within the housing having a portion adjacent the open end thereof; a display element received in and closing the open end of the housing and in heat transfer relation with the electrical resistance lamp element including: a first optically transmissive substrate having an optically transmissive and electrically conductive coating on a face thereof, a second substrate having an electrically conductive coating on a face thereof, the substrates being positioned such that the coatings are adjacent and substantially parallely spaced, a liquid crystal composition occupying the space between the coatings on the substrates, the composition having a threshold voltage level at which an optical characteristic of the composition is changed; and voltage impressing means for impressing a voltage gradient between the coatings, the sensor being electrically coupled to the means for impressing a voltage gradient so that current is conducted through the sensor, the sensor normally operating in one of two temperature ranges, the resistance level of the sensor controlling the magnitude of the voltage gradient, the voltage impressing means including means to electrically energize the electrical resistance lamp element, heat from the lamp element being transferred to the display element to provide ambient temperature compensation.
 2. Apparatus according to claim 1 in which the liquid crystal is nematic and the optical characteristic which changes upon application of threshold voltage level is a light transmitting characteristic whereby light passing therethrough is relatively unimpeded when the voltage level is below the threshold and is dynamically scattered when the voltage level is at least as high as the threshold level.
 3. Apparatus according to claim 2 in which the sensor comprises a thermistor formed of material having a positive temperature coefficient of resistivity, the sensor having a first low resistance state at temperatures below an anomaly and a second high resistance state at temperatures above the anomaly.
 4. Apparatus according to claim 2 in which one of the substrates is etched with a desired symbol.
 5. Apparatus according to claim 3 in which the sensor is electrically coupled in series with the display element, and a resistor is coupled across the display element to permit current to flow to the sensor from the voltage impressing means.
 6. Apparatus according to claim 3 in which the sensor is electrically coupled across the display element.
 7. Apparatus according to claim 3 further including a fluid reservoir, the sensor mounted in the reservoir such that it is completely immersed in the fluid when the reservoir is filled but is exposed when the fluid level falls beyond a predetermined limit.
 8. Apparatus according to claim 2 in which one of the conductive layers of the display element is formed in the shape of a desired symbol.
 9. A display unit comprising: a housing having an open end; an electrical resistance lamp element mounted within the housing having a portion adjacent the open end thereof; an optically transmissive lens received in and closing the open end of the housing and in heat transfer relation with the electrical resistance lamp element, the lens formed of a first and second parallely spaced optically transmissive substrates, the substrates each having an optically transmissive and electrically conductive coating on a face thereof, a liquid crystal composition occupying the space between the substrates, the composition having a threshold voltage level at which an optical characteristic of the composition is changed; and voltage impressing means connectable to the coatings and to the electrical resistance lamp element for impressing a voltage gradient between the coatings and for permitting current to flow through the electrical resistance lamp element, heat from the lamp element being transferred to the lens to provide ambient temperature compensation.
 10. A display unit according to claim 9 in which the liquid crystal composition is a temperature sensitive nematic composition.
 11. Apparatus according to claim 9 further including a sensor electrically coupled to the lens coatings, the sensor comprising a thermistor whose resistance varies with temperature.
 12. Apparatus according to claim 11 in which the thermistor is formed of material having a positive temperature coefficient of resistivity, the thermistor having a first low resistance state at temperatures below an anomaly and a second high resistance state at temperatures above the anomaly.
 13. Apparatus according to claim 11 in which the thermistor is formed of material having a negative temperature coefficient of resistivity. 